The present invention relates to heat transfer element assemblies and, more specifically, to an assembly of heat absorbent plates for use in a heat exchanger wherein heat is transferred by means of the plates from a hot heat exchange fluid to a cold heat exchange fluid. More particularly, the present invention relates to a heat exchange element assembly adapted for use in a heat transfer apparatus of the rotary regenerative type wherein the heat transfer element assemblies are heated by contact with the hot gaseous heat exchange fluid and thereafter brought in contact with cool gaseous heat exchange fluid to which the heat transfer element assemblies gives up its heat.
One type of heat exchange apparatus to which the present invention has particular application is the well-known rotary regenerative heat exchanger. A typical rotary regenerative heat exchanger has a cylindrical rotor divided into compartments in which are disposed and supported spaced heat transfer plates which, as the rotor turns, are alternately exposed to a stream of heated gas and then upon rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. As the heat transfer plates are exposed to the heated gas, they absorb heat therefrom and then, when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heated gas by the heat transfer plates is transferred to the cooler gas. Most heat exchangers of this type have their heat transfer plates closely stacked in spaced relationship to provide a plurality of passageways between adjacent plates for the flow of the heat exchange fluids therebetween. This requires means associated with the plates to maintain the proper spacing.
The heat transfer capability of such a heat exchanger of a given size is a function of the rate of heat transfer between the heat exchange fluids and the plate structure. However for commercial devices, the utility of a device is determined not alone by the coefficient of heat transfer obtained, but also by other factors such as cost and weight of the plate structure. Ideally, the heat transfer plates will induce a highly turbulent flow through the passages therebetween in order to increase heat transfer from the heat exchange fluid to the plates while at the same time providing relatively low resistance to flow through the passages and also presenting a surface configuration which is readily cleanable.
To clean the heat transfer plates, it has been customary to provide soot blowers which deliver a blast of high pressure air or steam through the passages between the stacked heat transfer plates to dislodge any particulate deposits from the surface thereof and carry them away leaving a relatively clean surface. This also requires that the plates be properly spaced to allow the blowing medium to penetrate into the stack of plates.
One method for maintaining the plate spacing is to crimp the individual heat transfer plates at frequent intervals to provide notches which extend away from the plane of the plates to space the adjacent plates. This is often done with bi-lobed notches which have one lobe extending away from the plate in one direction and the other lobe extending away from the plate in the opposite direction. Heat transfer element assemblies of this type are disclosed in U.S. Pat. Nos. 4,396,058 and 4,744,410. In the patent, the notches extend in the direction of the general or bulk heat exchange fluid flow, i.e., axially through the rotor. In addition to the notches, the plates are corrugated to provide a series of oblique furrows or undulations extending between the notches at an acute angle to the flow of heat exchange fluid. The undulations on adjacent plates extend obliquely to the line of bulk flow either in an aligned manner or oppositely to each other. These undulations tend to produce a highly turbulent flow. Although such heat transfer element assemblies exhibit favorable heat transfer rates, the presence of the notches extending straight through in the direction of bulk flow provides significant flow channels which by-pass or short circuit fluid around the undulated, main areas of the plates. There is a higher flow rate through the notch areas and a lower flow rate in the undulated areas which tends to lower the rate of heat transfer.
An object of the present invention is to provide an improved heat transfer element assembly wherein the thermal performance is optimized to provide an improved level of heat transfer, a desired plate spacing and a reduced quantity of plate material. In accordance with the invention, the heat transfer plates of the heat transfer element assembly have oblique undulations to increase turbulence and thermal performance but they do not have the axially extending, straight through notches for plate spacing. Instead, at least every other plate contains locally raised portions or dimples of a height to properly space the plates. The dimples are formed by drawing or stretching the material locally reducing the amount of plate material compared to notched plates. The undulations on adjacent plates may extend in opposite directions with respect to each other and the direction of fluid flow.